Force a specific default lock screen image
This setting allows you to force a specific default lock screen image by entering the path (location) of the image file.
This setting lets you specify the default lock screen image shown when no user is signed in, and also sets the specified image as the default for all users
(it replaces the inbox default image).
To use this setting, type the fully qualified path and name of the file that stores the default lock screen image.
You can type a local path, such as C:\windows\web\screen\lockscreen.jpg or a UNC path, such as \\Server\Share\Corp.jpg.
This can be used in conjunction with the "Prevent changing lock screen image" setting to always force the specified lock screen image to be shown.
Note: This setting only applies to domain-joined machines, or unconditionally in Enterprise and Server SKUs.
At least Windows Server 2012, Windows 8 or Windows RT
In Registry Editor we can see the following changes:
For Enabled we get:
FullKeyPath : HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\Personalization
ValueName : LockScreenImage
Value : C:\windows\web\screen\lockscreen.jpg